1. Field of the Invention
The present invention relates to imaging lenses which form an image of an object on a CCD sensor or C-MOS sensor used in a compact image pickup device and more particularly to image pickup devices with an imaging lens which are mounted in mobile terminals such as mobile phones and smart phones which are becoming increasingly compact and thin, and PDAs.
2. Description of the Related Art
In recent years, personal digital assistants (PDAs) and smart phones as mobile phones with a computer function have become more popular than mobile phones mainly designed for calls. The imaging lenses which are mounted in these mobile terminals and smart phones are expected to be small and thin, have a small F-value and provide high resolution and a wide angle of view. Conventionally an imaging lens composed of four lenses has been known U.S. Pat. No. 7,826,149, B2 (Patent Document 1) and JP-A-2004-341512 (Patent Document 2). As the image sensor size is becoming smaller, the demand for a smaller and thinner imaging lens is growing. One indicator of the compactness of an imaging lens is the ratio of total track length to maximum image height. It may be thought that the smaller this ratio is, the smaller the lens system is in the optical axis direction. Patent Document 1 discloses an imaging lens including, in order from the object side, a first lens with positive refractive power having a convex object-side surface, an aperture stop, a second lens with negative refractive power as a double-sided aspheric lens, a third lens with positive refractive power as a double-sided aspheric meniscus lens having a concave object-side surface or a double-sided aspheric biconvex lens, and a fourth lens with negative refractive power as a double-sided aspheric biconcave lens, in which the image-side surface of the fourth lens has a pole-change point and the maximum ratio between the distance on the optical axis from the aperture stop to the image plane and the total track length is defined. In this imaging lens, in order to shorten the total track length, the refractive power of the first lens is increased and the object-side surface of the fourth lens is concave so that the image-side principal point in the optical system is away from the image plane. In the imaging lens described in Patent Document 1, the ratio of total track length (TTL) to maximum image height (IH) (TTL/(21H)) is about 0.9 and the ratio of total track length to focal length is about 1.2 to 1.3, so a relatively high compactness is achieved. However, the imaging lens disclosed in Patent Document 1 assumes that the image sensor size is ¼ inch or more and the total track length is about 4 mm. If the above refractive power and lens surface combination is used to achieve further compactness, it would be difficult for each constituent lens to have an adequate center thickness and an adequate edge thickness. When the constituent lenses are manufactured by injection molding, this would pose a problem that resin filling work may be difficult. It is structurally impossible to apply this imaging lens, for example, to a small image sensor of ⅕ inch or less while keeping the ratio of total track length to maximum image height low. Patent Document 2 discloses an imaging lens including, in order from the object side, a positive meniscus lens having a convex object-side surface as a first lens, an aperture diaphragm, a meniscus lens having a convex image-side surface as a second lens, a positive meniscus lens having a convex image-side surface as a third lens, and a negative lens as a fourth lens, in which at least one surface of the fourth lens is aspheric and the ratio of the power of the paraxial region of the fourth lens to its power at maximum light ray height and the difference between the third lens' Abbe number and the fourth lens' Abbe number are set within appropriate ranges respectively. In this imaging lens, the first lens is a meniscus lens whose object-side surface has a strong positive power and the image-side principal point of the first lens is nearer to the object and the image-side surface of the fourth lens is concave, so that the total track length is short. In the imaging lens described in Patent Document 2, the ratio of total track length to maximum image height (TTL/(21H)) is about 1.17 and the ratio of total track length to focal length is about 1.5, so sufficient compactness is not achieved. In addition, the half angle of view is about 33 degrees, so the angle of view is not sufficiently wide.